I think the point here is that the "lenient" behaviour actually is a lot more complex than one would expect - a compliant mail server would need some special logic to detect hostnames that kinda look like IP addresses and handle them in a special way - without this behaviour being in any way standardized.
E.g., the lenient server would have to treat the value "10.0.0.254" like an IP address but "10.0.0.256" like a hostname that requires DNS resolution.
E.g., the lenient server would have to treat the value "10.0.0.254" like an IP address but "10.0.0.256" like a hostname that requires DNS resolution.